1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to an LCD device using cholesteric liquid crystal and a manufacturing method thereof.
2. Discussion of the Related Art
Flat panel display (FPD) devices having small size, lightweight, and low power consumption have been a subject of recent research according to coming of the information age. Among many kinds of FPD devices, LCD devices are widely used for notebook personal computers (PCs) or desktop PCs because of their excellent characteristics of resolution, color display and display quality. Generally, in an LCD device, first and second substrates having respective electrodes are disposed to face each other with a liquid crystal layer is interposed therebetween. The liquid crystal layer has an optical anisotropy due to an electric field generated by applying a voltage to the respective electrodes. The LCD device displays images by using a transmittance difference according to the optical anisotropy of the liquid crystal layer.
FIG. 1 is a schematic cross-sectional view of a related LCD panel.
In FIG. 1, first and second substrates 10 and 20, referred to as lower and upper substrates, are facing and spaced apart from each other. A thin film transistor “T” (TFT) having a gate electrode 11, and source and drain electrodes 15a and 15b are formed on an inner surface of the first substrate 10. The TFT “T” further has an active layer 13 and an ohmic contact layer 14. A gate insulating layer 12 is formed on the gate electrode 11. A passivation layer 16 is formed on the TFT “T”. The passivation layer 16 covers the TFT “T” and has a contact hole 16c exposing the drain electrode 15b of the TFT. A pixel electrode 17 is formed on the passivation layer 16 and connected to the drain electrode 15b through the contact hole 16c. 
A black matrix 21 is formed on an inner surface of the second substrate 20 at a position corresponding to the TFT “T”. A color filter layer 22a and 22b, in which colors of red (R), green (G) and blue (B) are alternately repeated, is formed on the black matrix 21. A common electrode 23 of transparent conductive material is formed on the color filter layer 22a and 22b. The color filter layer 22a and 22b of a single color corresponds to the one pixel electrode 17.
A liquid crystal layer 30 is interposed between the pixel and common electrodes 17 and 23. When a voltage is applied to the pixel and common electrodes 17 and 23, the arrangement of molecules of the liquid crystal layer 30 changes according to an electric field generated between the pixel and common electrodes 17 and 23. Orientation films (not shown) respectively formed on the pixel and common electrodes determine an initial arrangement of liquid crystal molecules.
First and second polarizers 41 and 42 are formed on outer surfaces of the first and second substrates 10 and 20, respectively. The first and second polarizers 41 and 42 convert natural light to linearly polarized light by transmitting only light whose polarizing direction is parallel to a transmission axis of the polarizer. The transmission axis of the first polarizer 41 is perpendicular to that of the second polarizer 42.
In FIG. 1, the TFT and the pixel electrode are formed on the lower substrate and the color filter layer and the common electrode are formed on the upper substrate. Recently, however, structures in which the TFT and the color filter layer are formed on the lower substrate, or the color filter layer and the common electrode are formed on the lower substrate and the TFT and the pixel electrode are formed on the upper substrate have been suggested.
Since an LCD device does not emit light for itself, an additional light source is necessary. Therefore, a backlight is disposed over the first polarizer 41 of FIG. 1 and light from the backlight is provided to a liquid crystal panel. Images are displayed by adjusting the light according to the arrangement of the liquid crystal layer. The LCD device of this structure is referred to as a transmissive LCD device. The pixel electrode 17 and the common electrode 23, two electrodes generating an electric field, are made of transparent conductive material and the first and second substrates 10 and 20 are also transparent.
Since only one polarizing component of the incident light is transmitted through the polarizer used in the LCD device and the other components are absorbed and then converted into heat loss, brightness of the LCD device is reduced by more than 50% considering reflection at a surface of the polarizer. To improve the brightness of the LCD device by reducing the heat loss, an LCD device having a reflective circular polarizer under the device is suggested. The circular polarizer transmits one circular polarizing component of the incident light and reflects the other components. The reflected circular polarizing components are reflected again by several optical parts under the circular polarizer and converted into a light component capable of passing the circular polarizer. Theoretically, since all the incident light is converted into one component and then transmits through the circular polarizer, loss of light occurring in a conventional linear polarizer is remarkably reduced.
FIG. 2 is a schematic cross-sectional view of a related art LCD device.
In FIG. 2, a first polarizer 42 that is a linear polarizer is disposed under a liquid crystal cell 41, in which a liquid crystal layer is interposed between two substrate having respective electrodes on inner surfaces. A retardation layer 43, which converts linear polarization into circular polarization and vice versa, and a second polarizer 45 that is a linear polarizer are disposed under the first polarizer 42. A compensation film 44 can be interposed between the retardation layer 43 and the second polarizer 45. A sheet 46 for collecting and diffusing light and a backlight 47 are sequentially disposed under the second polarizer 45. On the other hand, a third polarizer 48 whose transmission axis is perpendicular to that of the first polarizer 42 is disposed over the liquid crystal cell 41. The liquid crystal cell 41 can have the same structure as or different structure from the liquid crystal cell of FIG. 1.
The second polarizer 45 can be made through forming a cholesteric liquid crystal layer 45b on a transparent substrate 45a. The cholesteric liquid crystal has a selective reflection property that only light of a specific wavelength is selectively reflected according to a helical pitch of the molecules of the cholesteric liquid crystal. The polarization of the reflected light is determined according to a rotational direction of the liquid crystal. For example, if a liquid crystal layer has a left-handed structure where liquid crystal molecules rotate counter clockwise along a rotational axis, only left-handed circularly polarized light having a corresponding color, i.e., wavelength, is reflected. Since the pitch of the cholesteric liquid crystal that light experiences is varied according to an incident angle, a wavelength of reflected light is also varied. Accordingly, there is a color shift such that a color of transmitted light varies according to a viewing angle. To compensate for the color shift, a compensation film 44 may be disposed over the second polarizer 45.
As shown in FIG. 2, a sheet 46 for collecting light from the backlight 47 and diffusing light to the liquid crystal cell 41 can be disposed between the second polarizer 45 and the backlight 47.
In the LCD device of FIG. 2, brightness is improved through increasing transmitted light by using a circular polarizer to a conventional LCD device. However, a conventional linear polarizer is still necessary because polarizing efficiency of the circular polarizer is lower than that of the linear polarizer. Moreover, a retardation layer should be attached for light that passes the circular polarizer to transmit through the linear polarizer. Therefore, production cost is high due to a plurality of films required for an increase in brightness. However, the increase in brightness is not large and a viewing angle is narrow.
On the other hand, a LCD device using a cholesteric liquid crystal color filter (CLC) has been researched and developed recently. Since cholesteric liquid crystal has a selective reflection property, brightness can be improved in contrast with a LCD device using a color filter of absorption type.
FIG. 3 is a cross-sectional view of a related art LCD device using a CLC.
In FIG. 3, a circular polarizer 53 using cholesteric liquid crystal is disposed under a liquid crystal cell 51 having a CLC color filter 52. A collection sheet 54 and a backlight 55 are sequentially disposed under the circular polarizer 53.
A diffusing sheet 56 for diffusing light transmitted through the liquid crystal cell 51 is disposed over the liquid crystal cell 51. A retardation layer 57 and a linear polarizer 58 are sequentially disposed over the diffusing sheet 56.
The collection sheet 54 for collecting light entering the circular polarizer 53 and the CLC 52 is made by forming a film 54b having a high condensing pattern on a transparent substrate 54a. The collection sheet 54 may be made of only the film 54b without the substrate 54a. Moreover, the backlight 55 may include the high condensing pattern or means.
In a LCD device having the structure of FIG. 3, a wavelength variation of reflected light according to an incident angle to the cholesteric liquid crystal is solved by using a high condensing backlight and a collection sheet. Moreover, the light efficiency increases by using a circular polarizer and a reflective CLC, and the collected light is diffused through a diffusing layer over the liquid crystal cell. Therefore, the brightness is improved in contrast with a related art LCD device of FIG. 2, and the problem of a color shift according to a viewing angle is solved. However, the production cost and the thickness of the LCD device also increase due to the individual circular polarizer and collection sheet.